Four-piece infrared single wavelength lens system

Abstract

A four-piece infrared wavelength lens system includes, in order from the object side to the image side: a first lens element with a positive refractive power, a stop, a second lens element with a refractive power, a third lens element with a positive refractive power, a fourth lens element with a negative refractive power, wherein a focal length of the first lens element and the second lens element combined is f12, a focal length of the third lens element and the fourth lens element combined is f34, and they satisfy the relation: 0.2<f12/f34<1.4. Such a system has a wide field of view, large stop, short length and less distortion.

What is claimed is: 
     
       1. A four-piece infrared wavelength lens system comprising a stop and a lens group having four lens elements, in order from an object side to an image side, comprising:
 a first lens element with a positive refractive power, having an object-side surface being convex near an optical axis and an image-side surface being concave near the optical axis, at least one of the object-side surface and the image-side surface of the first lens element being aspheric; 
 the stop; 
 a second lens element with a positive refractive power, having an object-side surface being convex near an optical axis and an image-side surface being concave near the optical axis, at least one of the object-side surface and the image-side surface of the second lens element being aspheric; 
 a third lens element with a positive refractive power, having an object-side surface being concave near the optical axis and an image-side surface being convex near the optical axis, at least one of the object-side surface and the image-side surface of the third lens element being aspheric; 
 a fourth lens element with a negative refractive power, having an object-side surface being convex near the optical axis and an image-side surface being concave near the optical axis, at least one of the object-side surface and the image-side surface of the fourth lens element being aspheric; 
 wherein a focal length of the third lens element is f3, a focal length of the fourth lens element is f4, a central thickness of the third lens element along the optical axis is CT3, a central thickness of the fourth lens element along the optical axis is CT4, a radius of curvature of the object-side surface of the third lens element is R5, a radius of curvature of the image-side surface of the third lens element is R6, and they satisfy the relation: −0.91≤f3/f4≤−0.35, 0.5<CT3/CT4≤0.85, and 1.5<R5/R6<5.2. 
 
     
     
       2. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the first lens element is f1, a focal length of the second lens element is f2, and they satisfy the relation: 1.29≤f1/f2<1.6. 
     
     
       3. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the second lens element is f2, the focal length of the third lens element is f3, and they satisfy the relation: 1.31≤f2/f3<2.7. 
     
     
       4. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the first lens element is f1, the focal length of the third lens element is f3, and they satisfy the relation: 1.6<f1/f3<3.5. 
     
     
       5. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the second lens element is f2, the focal length of the fourth lens element is f4, and they satisfy the relation: −2.5<f2/f4≤−0.67. 
     
     
       6. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the first lens element is f1, a focal length of the second lens element and the third lens element combined is f23, and they satisfy the relation: 1.8<f1/f23<4. 
     
     
       7. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the second lens element and the third lens element combined is f23, the focal length of the fourth lens element is f4, and they satisfy the relation: −1.0<f23/f4<−0.2. 
     
     
       8. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the four-piece infrared wavelength lens system is f, a distance from the object-side surface of the first lens element to an image plane along the optical axis is TL, and they satisfy the relation: 0.6<f/TL<0.9. 
     
     
       9. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a radius of curvature of the object-side surface of the fourth lens element is R7, a radius of curvature of the image-side surface of the fourth lens element is R8, and they satisfy the relation: 1.3<R7/R8<5.0. 
     
     
       10. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a central thickness of the first lens element along the optical axis is CT1, a central thickness of the second lens element along the optical axis is CT2, and they satisfy the relation: 0.7<CT1/CT2<1.5. 
     
     
       11. The four-piece infrared wavelength lens system as claimed in  claim 1 , wherein a focal length of the first lens element and the second lens element combined is f12, a focal length of the third lens element and the fourth lens element combined is f34, and they satisfy the relation: 0.2<f12/f34<1.2.